In the first generation of single mode optical transmission networks, low loss single mode optical fibers were deployed in point-to-point links with directly modulated single mode lasers. A laser is directly modulated when its drive current is directly subjected to the modulated signal. For intercity transmissions where long repeater spacing and high bit rate communications are of primary concern, this is an effective and efficient use of single mode technology.
However, for short haul communications, the typical distance between a central office or other communications center and the user station is much shorter than the maximum available repeater spacing. In this environment, the use of a directly modulated laser for each communications link between a central office and an end user represents a considerable waste of laser power.
One way to more efficiently use single mode lasers and single mode optical fibers in a short haul system is to share a single laser among a plurality communications link. Laser sharing schemes for use in single short haul optical transmission systems are disclosed in U.S. patent application Ser. No. 680,398 filed on behalf of S. S. Cheng J. Lipson and S. D. Personick and U.S. patent application Ser. No. 732,556 filed on behalf of S. D. Personick. Both of these applications are assigned to the assignee hereof and are incorporated herein by reference.
In the systems described in the aforementioned patent applications, bidirectional communication is established between a central office and a plurality of user stations by means of two lasers operating at different wavelengths and located in the central office. The output of each laser is divided over a plurality of optical fibers connecting the central office with the user stations so that each fiber simultaneously transmits the two wavelengths from the central office to the user station. The first wavelength has information modulated onto it by means of external modulation while the second wavelength is transmitted unmodulated. Information which is modulated on the first wavelength is detected at the user stations while the second wavelength is externally modulated at the user stations and retransmitted to the central office.
Thus, the systems described in the above-mentioned patent applications utilize single mode technology to transmit information from the central office to the user stations and from the user stations back to the central office. While the single mode technology is necessary to transmit very wide bandwidth services such as high resolution digital video from the central office to the user stations, data transmissions from the user stations to the central office often need only lower bandwidth capability. Accordingly, simpler, non-single mode technology may be utilized to provide communications between the user stations and the central office.
It is the object of the present invention to provide a short haul communications system which enables single mode communication from a central office to a plurality of user stations while at the same time providing simpler and cheaper non-single mode technology for communications from the user stations to the central office.